The Literal Commandment
Also you shall not have intercourse with any animal to be defiled with it, nor shall any woman stand before an animal to mate with it; it is a perversion. (Leviticus 18:23)
The literal commandment is that a woman mate with an animal.
Messiah Says
Messiah implicitly affirmed this commandment when He spoke about the Law:
"Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill. For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished. Whoever then annuls one of the least of these commandments, and teaches others to do the same, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven. (Matthew 5:17-19)
Pictures of Messiah
The physical intimacy of sexual union between a husband and wife provides a physical picture of the spiritual intimacy that is supposed to exist between G-d and man and between Messiah and the bride of Messiah (the assembly of believers).
When that physical intimacy is "defiled" (to use the words of the commandment) by bringing an animal into the place of an honored husband then that picture is defiled and the message is corrupted. The message is twisted into "it is acceptable to have spiritual intimacy with something rather than the someOne with whom it is intended"... and that message is wrong.
That was the corrupted message of the Egyptian pantheon who had frog-headed goddesses and eagle-headed gods. It was out of this error that G-d drew His people, Israel, and brought them to Sinai and betrothed them with an everlasting covenant and taught them His good and holy ways.
G-d is a G-d of order and structure and we are not to violate the order He puts in place.
The Messiah is for salvation. There is no other part of creation intended for that purpose. Trying to find salvation in an animal-headed god is like trying to have sexual relations with that animal: G-d says "no".
How Messiah Fulfilled
Messiah was unable to fulfill this commandment because He is not a woman.
Traditional Observance
The traditional observance of this commandment is found in its literal sense: women not having sexual relations with an animal.
Other Notes
We are able to fulfill this commandment today and women should not have sexual relations with an animal.
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